1. Field of the Invention
The present invention relates to apparatus and methods for making underwater connections. More particularly, the present invention pertains to marine riser assemblies with controlled buoyancy, riser assemblies with pre-tensioned liners, and to methods of assembling and using such riser assemblies in underwater drilling operations.
2. Description of the Prior Art
A marine drilling riser is a conductor pipe used in offshore drilling operations for oil and gas. It is installed between the well site on the underwater floor and the floating drilling vessel or semi-submersible unit. The purpose of the riser is to guide the drill string to and from the well site, and to provide means for circulation of drilling fluid.
The riser constitutes a tubular column. A flexible joint is used to connect the riser to the well site, but generally does not support the riser. If the riser is required to support its own underwater weight, the riser could buckle or bend in deep water situations where a significantly long riser is used. Increasing the diameter of such a riser gives it added strength, but also increases the riser's cross section to currents and wave action.
Various types of tensioners have been proposed and/or employed in attempting to maintain a constant upward pull on the risers to relieve some of the weight supported by them. Since the tensioning devices are attached to the vessel, its heave must be compensated by the tensioners. However, in practice, fluctuations in such upward pull occur as the drilling vessel rises and falls in response to wave action, and the tensioners, whether mechanical or pneumatic, are not always able to respond to such vessel motion rapidly enough to maintain a constant force on the riser. In theory, both tension and position of the top of the riser are kept constant; but since there is no point of reference available, it cannot be determined whether the top of the riser actually remains still. In rough weather both the water surface and the vessel move independently, and even completely constant tension on the tension devices is no guarantee of lack of riser movement. For risers of three or four hundred feet in length, a few feet of movement can move the riser from a completely top-supported condition to a "squatted" condition in which the riser is bottom supported, and subject to dangerous buckling and failure.
Another disadvantage of relying solely on tensioners to support the riser is that the supporting force is applied only at the top of the riser. Since the riser is never maintained absolutely vertical, the tensioners should support the weight of the riser plus the weight of the drilling fluid less the weight of the water displaced by the riser. The drilling fluid is distributed along the length of the riser. Consequently, the net buckling load at every point on the riser is generally the sum of the weight of the riser below that point and the weight of the drilling fluid above that point, less the weight of the total volume of water displaced. Additional loading on the riser occurs due to any equipment suspended within the riser and to increases in the density of the drilling fluid column. Then, with tensioners supporting the riser only at its top where they are anchored on the drilling vessel, the continuously distributed load on the riser can still cause the riser to buckle.
Buoyancy tanks may be attached at one or more points along the length of the riser to provide lift to the riser. However, such tanks added to the riser increase the cross section and, therefore, the resistance of the assembly to currents. Also, to obtain a distribution of increased buoyancy along the riser, multiple tanks must be provided. Furthermore, once these tanks are installed, the buoyancy provided by the tanks cannot be adjusted to suit environmental or other condition changes. Thus, the danger of a positive buoyancy coupled with a break in the riser cannot be met by reducing the buoyancy so as to prevent the sudden rise of the riser assembly and its possible collision with the drilling vessel.
An additional disadvantage of fixed-buoyancy riser assemblies is experienced when, in the event of a threatened storm or rough water conditions, the riser assembly is disconnected at the well site. However, with fixed, slightly negative buoyancy, the disconnected riser will then weave below the vessel, making manipulation of the riser difficult.
Foams may be utilized as floatation material to add buoyancy to a riser. However, such a technique is but another form of fixed buoyancy method. In addition, foams tend to take on water when subject to undersea pressures, thereby reducing their effectiveness.
U.S. Pat. No. 3,858,401 discloses a system of open-bottomed buoyancy chambers surrounding the riser at various levels. Gas under pressure is fed into each chamber to displace sea water therefrom. A separate valve, actuated to close when a predetermined level of water is reached in the respective chamber, controls the flow of gas into that chamber. Gas removing means, such as a bleed line, can be used to reduce the buoyancy of each chamber. These buoyancy chambers, like the buoyancy tanks described hereinbefore, present an enlarged cross section to currents, and, as longer risers are used, more such chambers are needed.
Since the load on the riser at any point depends on the drilling fluid weight, the load on the riser can be reduced at virtually all points by lowering the density and/or quantity of drilling fluid contained within the riser. U.S. Pat. No. 3,434,550 discloses a system whereby the drilling fluid circulating upwardly within the annular region between the riser body and the drill pipe contained therein may be aerated to lower the fluid density. In this manner, the hydrostatic head of the drilling fluid in that annular region may be reduced to lighten the load on the riser. However, the system of that patent employs one or more gas manifolds external to the riser, and exposed to the currents. Furthermore, to maintain a constant buoyancy of the riser, gas must continuously flow through the circulating drill fluid.
It is known that tension may be used to improve the stiffness of various structures. For example, an airplane wing may be provided with inner tension wires to improve the resistance of the wing to bending arising from lift acting on the wing. Also, many reinforced concrete piles are prestressed during the casting process. Thus, the inner reinforcing steel is left in tension, while the concrete comprising the bulk of the pile remains in compression, during and after the driving process.
In the drilling of offshore wells from stationary platforms, inner casing strings are now often suspended from below the mud line. However, at one time it was common to suspend all such casing from the top, as for landbased wells, thus throwing the outermost string extending down to the mud line into compression. As water depth increased, centralizers were placed between the outer string and the next inner string, which was in tension, to combat possible buckling by the compressed outer string. Thus, though the outer string may buckle a small amount, the buckling is arrested by the inner string, as with airplane wings and prestressed piles.